WiMAX – Broadband Wireless Access Ahmed Younus Technical University of Munich, Germany [email protected] Abstract - WiMAX (Wireless Interoperability for Microwave Access) is a forum that is responsible for the task of ensuring interoperability and conformance among the systems and solutions developed by various vendors based, on IEEE 802.16 standard for Broadband Wireless Access (BWA).This paper gives an introduction to the 802.16 working group’s history, WiMAX technology, its applications, features and a brief overview of the physical (PHY) layer and medium access control (MAC) layer. I. INTRODUCTION The IEEE Working Group 802.16 is responsible for the development of the 802.16 standard including the air interface for Broadband Wireless Access. The activities of this working group were initiated in a meeting in August 1998, called by National Wireless Electronics Systems Testbed (N-WEST) which is a part of U.S. National Institute of Standards and Technology. Initially the group focused on the development of standards and air interface for the 10-66 GHz band. Later an amendment project led to the approval of the IEEE 802.16a standard meant for 2-11 GHz band. The final approval of the 802.16a Air Interface specification came in January 2003. The WiMAX forum is a non profit organization that came into existence in July 2001. It comprises of Industry leaders who are committed to promote broadband wireless access based on WiMAX technology. This forum will define and conduct interoperability tests and award the equipment manufacturers with the ‘WiMAX Certified’ label. This ultimately would benefit the end users who can buy the brand of their choice, with all the features they are interested in and with the full confidence that this would work with all other certified products. II. Applications WiMAX technology offers higher bandwidth and greater range as compared to Wi-Fi (Wireless Fidelity - IEEE 802.11 Standard) based wireless systems. It is a wireless alternative to many existing wired backhaul and last mile coverage deployments such as Cable Modems, Digital Subscriber Line (DSL), T and E-Carrier systems and Optical carrier technologies. Its importance is obvious in areas which are sparsely settled, across difficult terrains and in areas where it is not feasible to run cables. Therefore this leads to inexpensive deployment of the network and ubiquitous broadband access. The IEEE 802.16 Working Group has developed standards that cater mainly to two types of usage models. IEEE 802.16 – Fixed users (Residential, SOHO, Enterprises) IEEE 802.16e – Portable users (Nomadic, Mobile) The typical deployment scenarios for WiMAX systems are shown in the figure 1. Backhaul: Point to Point antennas are used to connect BSs located across long distance

Last Mile: Residential and Business subscribers are connected to the Base Stations using Point-to-Multipoint antennas

Fig 1. Deployment Scenarios for WiMAX [5]

Large Area Coverage Access: WiMAX also offers broadband connectivity in larger areas (hotzones). Wi-Fi and WiMAX offer complementary solutions with Wi-Fi being suitable for short range and indoor connections and WiMAX being suitable for long range outdoor connections. III. System Performance WiMAX offers high data rates and extended coverage. •

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A maximum of 75 Mbps bandwidth is achievable with a 20 MHz channel under best channel conditions. However, the maximum bandwidth is reduced in the areas where regulators allow only narrower channels. The theoretical coverage radius is about 30 miles under optimal conditions and with a reduced data rate. At extremely long range, the data rate drops to 1.5 Mbps. The typical coverage with indoor Customer Premises Equipment (CPE) is about 5 Km and with a CPE connected to an external antenna (LOS), it is around 15 km. IV. WiMAX Physical (PHY) Layer

WiMAX PHY layer specification has been standardized keeping in view the 10-66 GHz (LOS) and 2-11 GHz (NLOS) bands. In the 10-66 GHz band, the LOS propagation is possible only since at such high frequencies, the signal suffers severe attenuation. The air interface for this band is termed as WirelessMAN-SC (single carrier). Due to the point to multipoint architecture of WiMAX systems, the base station (BS) transmits a TDM signal. The subscriber stations are allocated timeslots in a serial fashion. Support has been provided both for TDD and FDD. Half duplex FDD subscribers can also be accommodated. The TDD and FDD duplexing support addresses the varying worldwide regulations.

The 2-11 GHz band specification is given in the IEEE 802.16a standard. Three types of air interfaces are supported in this band: WirelessMAN SC2: uses a single carrier modulation format WirelessMAN OFDM: uses OFDM with 256 point transform WirelessMAN OFDMA: uses OFDM with 2048 point transform OFDM Technology: The WiMAX OFDM (Orthogonal Frequency Division Multiplexing) waveform removes the complexities of adaptive equalization and the problems of ISI (intersymbol Interference) due to multi-path delay spread. The OFDM waveform is composed of multiple narrowband orthogonal carriers. As a result, the effect of selective fading is localized to a subset of carriers which makes the process of equalization simple. Whereas in a single carrier system, a broader carrier signal needs to be equalized which requires complex adaptive equalization techniques.

Fig 2. Single Carrier and OFDM received signals [4]

The dotted area represents the transmitted spectrum and the solid area shows the receiver input. Sub Channelization: WiMAX supports sub channelization in the Uplink. In general, the CPE/MS is power constrained in any wireless network. This makes the link budget asymmetrical and the coverage area of a BS is uplink limited. Sub channelization balances the link budget in the uplink and downlink and extends the coverage area in a WiMAX system.

Fig 3. Effects of sub-channelization [4]

Flexible Channel Sizes: The flexible channel sizes supported by WiMAX PHY layer ensures worldwide deployment. The operating frequencies allowed by regulating authorities might vary from country to country. It also ensures the utilization of the available spectrum with the

maximum efficiency. If an operator has a 14 MHz band, it will not desire a system with 5 MHz channels as this will lead to wastage of 2 MHz of spectrum. This specially becomes critical when the network is operated in the licensed spectrum where the operator has paid a huge amount for the license. Advanced Antenna Techniques: WiMAX supports advanced antenna features to improve transmission. This can be done by using Antenna Diversity systems or using Smart Antennas. The diversity techniques employ multiple antennas in receivers or transmitters which provide alternate paths for the signal and reduce the effect of multipath fading. The smart antenna approach uses beam forming and steering mechanism in which the angle, path and width of the beam can be controlled. This helps to eliminate unwanted interference and increases the system capacity. Adaptive Modulation: Adaptive Modulation enables a WiMAX system to optimize the throughput based on the propagation conditions. Using Adaptive Modulation scheme, WiMAX system can choose the highest order modulation provided the channel conditions are good. As the SNR is very good near the BS, so higher order modulation scheme is used in this area to increase the throughput. However, in areas close to the cell boundary, the SNR is normally poor. So, the system steps down to a lower order modulation scheme to maintain the connection quality and link stability. The supported modulation rates are BPSK, QPSK, 16-QAM and 64-QAM.

Fig 4. Beam Shaping [7]

Fig 5. Adaptive modulation [7]

Forward Error Correction (FEC): Error correction techniques are also a key feature of WiMAX technology. Errors are detected and corrected using Reed Solomon FEC, convolutional encoding and interleaving algorithms. All these techniques help in recovering the errored frames that might have got corrupted due to frequency selective fading or burst errors. V. Medium Access Control (MAC) Layer The IEEE 802.16 MAC layer is responsible to provide a medium independent interface to the 802.16 PHY layer. Two types of service specific convergence sublayers are defined by 802.16 MAC. The ATM convergence sublayer is defined for ATM services and the packet convergence sublayer is defined for mapping packet based services like IPv4, IPv6, Ethernet etc. The 802.16 MAC is connection oriented. Each subscriber station has a 48-bit MAC address. Howerver, this MAC address merely serves as the equipment identifier as the connections are referenced with the 16-bit connection identifier (CID). MAC PDU Format: A MAC protocol data unit (PDU) consists of a fixed length header, a variable length payload and an optional field for cyclic redundancy check (CRC). The maximum length of a MAC PDU is 2048 bytes. There are two types of MAC headers: a generic header, which is used to transmit data or MAC messages and a bandwidth request header, which is used by the subscriber station (SS) to request more bandwidth on the UL.

Three types of MAC subheaders may also be present. The grant management subheader is used to convey bandwidth management needs to the BS. The fragmentation subheader indicates the presence and orientation of any fragments of service data units (SDU) in the payload. The packing subheader indicates the packing of multiple SDUs into a single PDU. MAC Connection Types: The 802.16 MAC also defines three types of MAC management connections that are used to exchange control messages between the BS and SS. These connections specify different QoS requirements, needed at different management levels. The basic connection is used to transfer short, time-critical MAC and Radio Link Control (RLC) messages. The primary management connection transfers long and delay tolerant messages related to authentication and connection setup. The secondary management connection is used to transfer standards-based messages like Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP) and Simple Network Management Protocol (SNMP). Network Entry State Machine: A WiMAX subscriber station has to complete the network entry process shown in the figure 6, in order to communicate on the network. The network entry state machine moves to reset if it fails to succeed from any state. a)Downlink Channel Synchronization: When an SS wants to communicate on a WiMAX network, it first scans for available channels in the defined frequency list. On finding a DL channel, it tries to synchronize at the PHY level using the periodic frame preamble. Information on modulation and other DL and UL parameters is obtained by observing the DL Channel Descriptor (DCD) and the UL channel descriptor (UCD) of the DL channel. b) Initial Ranging: An SS starts an Initial ranging process by sending a ranging request MAC message using the minimum transmission power. If no Fig 6. Network Entry Process [2] response is received from the BS, the SS resends the message on a subsequent frame using a higher transmission power. The response either indicates power and timing corrections that the SS must make or indicates success. c) Exchanging Capabilities: After successful completion of the initial ranging step, the SS sends capability request message indicating the supported modulation level, coding scheme and rates and duplexing methods. d) Authentication: After capability negotiation, the BS authenticates the SS, determines the ciphering algorithm to be used, and sends an authentication response to the SS.

e) Registration: After authentication, the SS sends a registration request message to the BS and the BS sends a registration response. f) IP Connectivity: After registration, the SS gets the IP address via DHCP. The SS also downloads other operational parameters using TFTP. g) Connection Creation: After completing the IP connectivity step, transport connections are created. For preprovisioned service flows, the BS sends a dynamic service flow addition request message to the SS and SS confirms the creation of connection. For non-preprovisioned service flows, connection creation is initiated by the SS by sending a dynamic service flow addition request message to the BS. The BS responds with the confirmation. Service Classes: The 802.16 MAC specifies four different types of service classes in order to provide QoS for various types of applications. • • • •

Unsolicited Grant Service (UGS): supports constant bit rate (CBR) services such as T1/E1 emulation and VoIP without silence suppression. Real-Time Polling Services (rtPS): supports real-time services that generate variable size data packets on periodic basis like MPEG video or VoIP with silence suppression. Non Real-Time Polling Services (nrtPS): supports non real time services requiring variable size data grant burst types on regular basis. Best Effort (BE) Services: supports web surfing over the internet. VI. Conclusion

WiMAX technology based on IEEE 802.16 standard provides a platform for the deployment of metropolitan area networks (MAN) providing broadband wireless access. WiMAX forum will ensure that multiple vendors produce interoperable equipment. It will help in bridging the digital divide and will ensure faster roll out in areas where the low POTS penetration, high DSL costs or poor copper quality have acted as a barrier in providing high speed, broadband internet access. To address high speed mobility issues, the IEEE 802.20 Mobile Broadband Wireless Access (MBWA) working group has already been formulated. The mission of this group is to develop an air interface for MBWA systems operating below 3.5 GHz with peak user data rates of 1 Mbps, upto vehicular speed of 250 km/h. The WiMAX certified products are available in the market and commercial WiMAX networks are expected in the first quarter of 2006. In short, WiMAX will turn the dream of ubiquitous broadband wireless access into reality very soon. VII. References [1] – “IEEE 802.16a Standard and WiMAX Igniting Broadband Wireless Access”, White Paper [2] – Govindan Nair, Joey Chou, Tomasz Madejski, Krysztof Pereycz, David Putzolu, Jerrz Szdir, “IEEE 802.16 Medium Access Control and Service Provisioning”, Intel Communications Group, Intel Corporation [3] – Carl Eklund, Roger B. Marks, Kenneth L. Stanwood, Stanley Wang, “IEEE Standard 802.16: A technical overview of the WirelessMAN Air Interface for Broadband Wireless Access”, IEEE Communications Magazine, June 2002. [4] – “WiMAX’s technology for LOS and NLOS environments”, WiMAX Forum [5] – “WiMAX, making ubiquitous high-speed data services a reality”, White Paper, Alcatel [6] – “Adaptive Modulation (QPSK, QAM)”, Application Notes, Intel Corporation [7] – “Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless”, Technical White Paper, Intel Corporation